FIG. 1 is a high level diagram of a conventional vector network analyzer (VNA) 100 which is shown as including a signal generator 102 (also known as a voltage source) having a source impedance RS, a forward coupler 104, a reverse coupler 106, one or more test port 108, a receive/detector 110 and a processor/display 120. The signal generator 102 produces signals that are transmitted to a test device, shown as a resistive load RL, which is connected to one or more test port 108 of the VNA. The resistive load RL, also known as a device under test (DUT), can be, e.g., an open, a short, or anything in-between.
The forward coupler, which includes a resistive bridge having an impedance RF, provides a forward signal (also known as an incident signal) to a first input of the receiver/detector 110. The reverse coupler 108, which includes a resistive bridge having an impedance RR, provides a reverse signal (also known as a reflected signal) to a second input of the receiver/detector 110. The forward signal is often referred to as signal VF hereafter. Similarly, the reverse signal is often referred to as signal VR hereafter. In the arrangement shown, the forward coupler 104 and the reverse coupler 106 together form what is known as a dual directional coupler. Preferably, the impedances of the source, the load and the couplers 104 and 106 all match (i.e., preferably RS=RL=RF=RR). A typical value for RS, RL, RF and RR is 50 ohms, or 75 ohms, but other values may be used.
The receiver/detector 110, which is tuned to the frequency of the signal generator 102, likely includes components such as a local oscillator (LO), band-pass filters (BPFs) and a synchronous detector or digital-signal processor (DSP). The LO is used to mix received RF signals down to lower intermediate frequency (IF) signals and baseband signals. The BPFs are used to filter out undesired harmonics and noise from the IF and baseband signals. The synchronous detector or DSP is used to extract magnitude and phase information from the baseband signals, which can be, e.g., about 200 KHz signals. More specifically, the synchronous detector or DSP can converts the VF and VR signals into digital signals indicative of real and imaginary components of the VF signal and real and imaginary components of the VR signal. The processor/display 120 formats the reflection and transmission data in ways that make it possible to interpret results of measurements.
Conventional forward and reverse couplers 104 and 106 typically have a low frequency limit of approximately 1 or 2 MHz. This is primarily due to the balun used to create the single ended to differential signal for the resistive bridge of the coupler. More specifically, a conventional coupler typically includes a coaxial transmission line surrounded by Ferrite beads of about a quarter inch in length. A coupler with one quarter inch Ferrite bead will enable use of the coupler down to about 2 MHz. To get down to about 1 MHz, two such quarter inch Ferrite beads are needed; to get down to about 0.5 MHz, four such quarter inch ferrite beads are required; and so on. Accordingly, it can be appreciated that it would be prohibitive to produce a coupler useful down to about DC, using the above described methodology. Nevertheless, there is a desire to measure S-parameters down to DC. Accordingly, there is a need for couplers that are useful down to DC.